The present invention relates to a DC-DC power source device for obtaining a prescribed DC power from DC power source.
FIG. 6 shows the configuration of an AC variable speed device with a conventional built-in DC-DC power source device. In FIG. 6, reference numeral 30a designates a three-phase AC power source; 31a, a converter unit which converts AC power to DC power; 32, a neutral point between electrode terminals P and N of a DC main circuit power source; 33, a P side smoothing circuit connected between the anode P and the neutral point 32 of the DC main circuit power source; and 34, an N side smoothing circuit connected between the cathode N and the neutral point 32 of the DC main circuit power source. Reference numeral 35 denotes a P side balancing circuit connected between the anode terminal P and the neutral point 32 of the DC main circuit power source; 36, an N side balancing circuit connected between the cathode terminal N and the neutral point 32 of the DC main circuit power source.
In addition, reference numeral 37a denotes an inverter unit that inverts DC power of the DC main circuit power source to AC power with a variable frequency and a variable voltage; 38a, an induction motor driven at a variable speed; 39a, a load circuit comprising the inverter unit 37a and the induction motor 38a and operates as a load of the DC main circuit power source.
Further, reference numeral 40 indicates a starting circuit connected between the anode terminal P of the DC main circuit power source and a switching control circuit 45; 41, a high-frequency transformer having two secondary windings; 42, a rectifier circuit diodes for generating a DC output current from the high-frequency transformer 41; and 43,a rectifier circuit capacitor.
Reference numeral 44 designates a DC rectifier circuit comprising the rectifier circuit diodes 42 and the rectifier circuit capacitor 43; 45, a switch control circuit; 46,a switching circuit; 47, a secondary rectifier circuit diode; 48, a secondary rectifier circuit capacitor; 49, a secondary load such as a control unit (not shown) for controlling the inverter unit 37a. 
The DC-DC power source device contained in the conventional AC variable speed device is comprised of the starting circuit 40, the high frequency transformer 41, the DC rectifier circuit 44, the switch control circuit 45, the switching circuit 46, the secondary rectifier circuit diode 47 and the secondary rectifier circuit capacitor 48.
The operation of the conventional AC variable speed device is set forth below. AC power of the three phase AC power source 30a is converted to DC power by the converter unit 31a and the converted DC power is then filtered by a smoothing circuit (the P side smoothing circuit 33 and the N side smoothing circuit 34) to work as the DC main circuit power source. The inverter unit 37a inverts DC power of the DC main circuit power source to AC power with variable frequency and variable voltage, thereby driving the induction motor 38a at variable speeds.
The conventional AC variable speed device contains the DC-DC power source device as DC power source of control unit (not shown) for controlling the inverter unit 37a, supplying a prescribed DC power source to the control unit by utilizing the DC main circuit power source. Here, the P side balancing circuit 35 and the N side balancing circuit 36 are used to regulate a voltage sharing ratio between the P side smoothing circuit 33 and the N side smoothing circuit 34 to lower the voltage applied to these smoothing circuits 33 and 34 below their withstand voltage.
It is assumed that these balancing circuits (P side balancing circuit 35 and N side balancing circuit 36) do not exist. Defining that an internal impedance of the P side smoothing circuit 33 is r1, an internal impedance of the N side smoothing circuit 34 is r2, a voltage applied to the P side smoothing circuit 33 is v1, a voltage applied to the N side smoothing circuit 34 is v2, and a DC main circuit voltage smoothed is V DC 1, the voltage v1 applied to the P side smoothing circuit 33 and the voltage v2 applied to the N side smoothing circuit 34 are expressed by the following formulas (1) and (2), respectively:
v1=(r1/(r1+r2))xc3x97VDC1xe2x80x83xe2x80x83(1)
v2=(r2/(r1+r2))xc3x97VDC1xe2x80x83xe2x80x83(2)
When the capacitance and withstand voltage V of the P side smoothing circuit 33 are identical with those of the N side smoothing circuit 34, and when the internal impedance r1 of the P side smoothing circuit 33 are 3 times larger than the internal impedance r2 of the N side smoothing circuit 34, the formulas (1) and (2) are expressed by following formulas (3) and (4), respectively:
v1=(xc2xe)xc3x97VDC1xe2x80x83xe2x80x83(3)
v2=(xc2xc)xc3x97VDC1xe2x80x83xe2x80x83(4)
Therefore, the withstand voltage V of the P side smoothing circuit 33 must be at least xc2xe times larger than that of the DC main circuit voltage VDC1. The balancing circuits (P side balancing circuit 35 and N side balancing circuit 36) are disposed in order to adjust the difference in voltage applied to the smoothing circuits, caused by the difference between the internal impedance r1 of the P side smoothing circuit 33 and the internal impedance r2 of the N side smoothing circuit 34.
Given now that, the impedance of the P side balancing circuit 35 is R21, the impedance of the N side balancing circuit 36 is R22, the combined resistance of the impedance R21 of the P side balancing circuit 35 and the internal impedance r1 of the P side smoothing circuit 33 is Rc21, and the combined resistance of the impedance R22 of the N side balancing circuit 36 and the internal impedance r2 of the N side smoothing circuit 34 is Rc22, the voltage v1 applied to the P side smoothing circuit. 33 and the voltage v2 applied to the N side smoothing circuit 34 are expressed by the following formulas (5) and (6),respectively:
v1=(Rc21/(Rc21+Rc22))xc3x97VDC1xe2x80x83xe2x80x83(5)
v2=(Rc22/(Rc21+Rc22))xc3x97VDC1xe2x80x83xe2x80x83(6)
When the impedance R21 of the P side balancing circuit 35 is assumed as R21 less than  less than r1, and when the impedance R22 of the N side balancing circuit 36 is assumed as R22 less than  less than r2, the combined resistances Rc21 and Rc22 are expressed as Rc21≈R21 and Rc22≈R22, respectively.
Assuming herein that R21=R22, the formulas (5) and (6) can be expressed by the following formulas (7) and (8), respectively.
v1=VDC1/2xe2x80x83xe2x80x83(7)
v2=VDC1/2xe2x80x83xe2x80x83(8)
The voltage v1 applied to the P side smoothing circuit 33 and the voltage v2 applied to the N side smoothing circuit 34 are xc2xd times larger than DC main circuit voltage VDC1, thereby making it possible to adjust the imbalance voltage applied to each smoothing circuit.
Therefore, the withstand voltages v of the P side smoothing circuit 33 and the N side smoothing circuit 34 are lowered to equal to or less than xc2xd of the DC main circuit voltage VDC1.
An operation of the DC-DC power source device contained in the conventional AC variable speed device will be described. Application of AC power source 30a to the AC variable speed device causes the DC main circuit power constituted by the converter unit 31a and the smoothing circuit (P side smoothing circuit 33 and N1 side smoothing circuit 34) to charge the rectifier circuit capacitor 43 through the starting circuit 40. Besides, the rectifier circuit capacitor 43 supplies the charged DC power to the switch control circuit 45, and as a result, the circuit 45 outputs a high frequency oscillating signal to the switching circuit 46. The switching circuit 46 oscillates at high frequency and supplies high frequency power to the high frequency transformer 41. The high frequency transformer 41 supplies DC power to the DC rectifier circuit 44 comprised of the rectifier circuit diode 42 and the rectifier circuit capacitor 43, and supplies DC power to the secondary rectifier circuit diode 47, the secondary rectifier circuit capacitor 48 and the secondary load 49.
Besides, after the power source is activated, operating power of the switch control circuit 45 is supplied from the direct current rectifier circuit 44 and the starting circuit 40.
In above conventional DC-DC converter, there is a problem that circuit loss increases due to the fact that a current from the DC main circuit power; source always flows through the starting circuit 40. The loss P in the starting circuit 40 can be expressed by the following formula (9), where the DC main circuit voltage is VDC1, the impedance of the starting circuit 40 is r3, and voltage across both terminals of the rectifier circuit capacitor 43 is VDC2.
P=(VDC1xe2x88x92VDC2)2/r3xe2x80x83xe2x80x83(9)
In the case where the input AC power source equal to or more than 420V, this loss increases further.
In order to secure distance sufficient for insulation, it is necessary to have an enough mounting area, presenting obstacles to miniaturization of the circuit.
It is an object of the invention to solve aforementioned problems.
Another object of the invention is to reduce the circuit loss in the DC-DC power source device.
It is a further object of the invention to miniaturize the DC main circuit power source device.
A DC-DC power source device of this invention is comprised of a secondary rectifier circuit constituted by a secondary rectifier diode and a secondary rectifier capacitor, a DC rectifier circuit formed by a DC rectifier diode and a DC rectifier capacitor, a switch control circuit for outputting a high frequency oscillating signal according to a DC power supplied from the DC rectifier circuit, a switching circuit for oscillating at a high frequency according to the high frequency oscillating signal outputted from the switching control circuit to generate high frequency power, a high frequency transformer having two secondary windings and supplying a DC power to the secondary rectifier circuit and the DC rectifier circuit according to high frequency power supplied from the switching circuit, wherein one end of the primary winding of the high frequency transformer is connected to an anode of a DC main circuit power source, the other end of the primary winding being connected to the switching circuit, wherein the anode of DC rectifier circuit supplying a DC power to the switching control circuit is connected to an anode of the DC main circuit power source through a balancing circuit including at least two balancing resistors which connected in series each other, wherein the balancing circuit is connected in parallel with a smoothing circuit which is connected to the anode of the DC main circuit power at one end and which comprises at least two capacitors with same capacitance being connected in series between the anode and the cathode of the DC main circuit power, and wherein a combined impedance of the balancing resistor connected to the cathode of the DC main circuit power and the switching control circuit is equal to that of the other balancing resistor.
In addition to the feature as recited above, the DC-DC power source device of this invention is further comprising a detour prevention circuit interposed between the DC rectifier capacitor and the balancing resistor connected to the cathode of the DC main circuit power source, a voltage applied to the switch control circuit being kept equal to or less than a predetermined value when the DC main circuit voltage becomes high.